High-frequency dielectric heating



8" 1951 R. B. PARMENTER ETAL 2,564,579

HIGH-FREQUENCY DIELECTRIC HEATING Filed March 8, 1946 2 Sheets-Sheet l A TTORNEVS Aug. 14, 1951 R. B. PARMENTER ETAL 2,564,579

HIGH-FREQUENCY DIELECTRIC HEATING Filed March 8, 1946 2 Sheets-Sheet 2 06,421 Mwmaw ATTORNEYS Patented Aug. 14, 1951 2,564,579 I CE 2,564,57 9 HIGH-FREQUEN CY DIELECTRIC HEATING Robert B. Parmenter, Louisville, Ky., and'Edgar L. Marven, Floyds Knobs, Ind., assignors to The Girdler Corporation, poration of Delaware Louisville, Ky., a cor- Application March 8, 1946, Serial No. 652,932

2 Claims. 1

Our invention relates to high-frequency heating systems of the type in which high-frequency electric fields are applied to dielectric materials to elevate their temperature and particularly relates to provision of systems suited readily to heat relatively small articles and materials such as cereals, grains and tie like comprising small,

separate particles.

It has for some time been understood that materials generally classified as electrical insulators may be heated by subjecting them to an alternating electric'field. According to accepted formula, the power input to, or loss in, a dielectric material varies directly with the frequency of the field, directly with the loss factor (the product of the power factor and dielectric constant of the material), and as the square of the voltage per unit thickness of the material.

However with relatively thin materials or objects of small size, there is definite limit to the voltage which may be applied because of danger of arcing between the electrodes: moreover for many materials the loss factor is very low. In

consequence, for heating of such objects and materials at relatively high rate, very high frequencies should be employed.

One use of dielectric heating, here of interest, is the rapid sterilization of a small quantity of liquid sealed in an anipule of glass: another con cerns chemical, biological, medical and other research work requiring the rapid heating of a small quantity of liquids, solids, or gas disposed in a bottle, test tube, or other glass container.

It has long been known that fiourymalt, wheat and other grains maybe heated by :high-frequency fields to destroy organic growth, worms, weevils and the like.

For these and like applicationsof dielectric heating, the problem of developing heat at'suit ably rapid rate has not:been heretofore satisfactorily solved. heated. requires electrodes of relatively small size,

Where the material to be it has been diificult to develop highzpower input to material disposed therebetween .-.for containers such as'test'tubes, the size of electrodesusedahas been so great the capacity-formed by them has limited :the range of frequency that could "be applied.

It is an object of our invention to'provide a dielectric heating system by means of which maximum permissible voltages arepro'duced at the electrodes and at frequencies of from .10 to invention, are slidable as a unit along the anode line of an oscillator to position-resulting'in application of maximum difference of radio-frequency voltage between the electrodes.

In a specific example of another-form of our ,irn ention, theelectrodes may .be mounted at-the open end of a parallel line inductively coupled ating frequency.

'trical length "of the line.

to an oscillator tank circuit and. tunable by a trombone slide so that its electrical length, longer than its physical length because of the electrical capacitance of the electrodes, is an odd number of quarter-wavelengths long at the oper- More particularly, to ensure relative uniform potential at the various points of the surface of each electrode, its greatest dimension should bemuch less than a quarter-wavelength and the point of its connection to the line should'be at'or near the center of the electrode.

Our invention further resides in systems'having features hereinafter more fully described.

For an understanding of our invention reference is made to the accompanying drawings in which:

Fig. 1 schematically illustrates a dielectric heatingsystem in which the heating electrodes are attached to theano'de line of an oscillator.

Figs. 2 and 3 are elevational and plan views respectively showing constructional details of'a heating-electrode assembly for the system of Fig. 1.

Figs. 4 and 5 are different views of'another -form of'electrode device adapted to be inductively coupled to'an oscillator.

Figs. 6 and 7 are elevation and plan views respectively of another electrode device providing for inductive coupling to an oscillator and having readily detachable electrodes.

Figs. 6A and 7A are elevation and plan views respectively 'of electrodes interchangeable with 'thoseshown in Figsifi and 7.

8 schematically illustrates another heating systemand includes a plan view of a modified form of electrode device.

device-of Fig. 8 in "tors Hand 24 in the tuned-line output circuit of a high-frequency oscillator.

The electrical length of this line may be one quarter-wavelength or any odd number of quarter-wavelengths so'that beginning at the-anode end of the line there are one or morepairs of voltage maxima or'anti-nodes depending on the elec- For a given frequency of operation, the electrode assembly is so positioned along "the line that electrodes IO and H are connected to conductors 23 and 24 at or adjacent points of maximum voltage-difference. Thus maximumfield intensity 'is obtained between the electrodes and since these electrodes conform with or 'fit'thecontainer, this field-is concentrated therein so affording'rapid heating of the dielectric-material in the container.

As shown in more detail in Figs. 2 and 3, the electrode assembly comprises the pair of electrodes I and II supported on a strip iii of insulation, preferably laminated mica or other material having a low loss factor, by angle pieces 14 and i5. To minimize corona and so permit utilization of high voltages, the edges of the electrodes are provided with, or formed to provide, the members [6 and I! of rounded cross-section. The electrodes Hi and II are mounted with their concave faces positioned and spaced to fit or conform with a test tube or the like which may be inserted between them. The spring clips 8 and 9, fastened to the underside of strip [3 and electrically connected to electrodes l8 and H respectively, are suited slidably to engage the conductors 23 and 24 and so permit the electrode assembly to be moved along the anode line to position affording optimum heating eifect.

In the particular form of oscillator shown in Fig. l, the tubes 2| and 22 are connected in pushpull with their anodes or plates connected to the parallel conductors 23 and 24 in the anode-line tank circuit. The anodes are connected to the positive terminal of a suitable source of directcurrent through radio-frequency choke coil 32 connected to the closed end of the line remote from the anodes. The grid circuit of the oscillator may also, as shown in Fig. 1, be a tuned line comprising elongated conductors 21 and 28 connected at the open end of the line to the grids of the tubes 2| and 22. The closed end of the grid line is connected through the biasing resistor 3i! and ammeter A to the cathodes of the tubes and to the negative terminal B- of the source of direct-current for the anodes.

The positions of the shorting bars 25 and 29 and of the electrode assembly are predetermined or adjusted to obtain the desired amplitude and frequency of the radio-frequency voltage applied to the heating electrodes Iii and Ii. The electrode device connected as above described to the anode line adds an appreciable amount both of lumped capacitance and of lumped inductance with the result that the anode line is somewhat shorter than its electrical length and so, for a given operating frequency, requires a different position of bar 25 than would otherwise be proper for resonance of the line at an odd number of quarter-wavelengths. For a given position of bar 25, the resonant frequency of the anode line is variable by adjustment of the electrode assembly: thus the desired voltage gradient between the electrodes and proper loading of the oscillator may be attained by adjustment of the electrode assembly along the anode line.

With this arrangement in which the heating electrodes also serve as lumped capacitance in an anode line, shifts in frequency, due to variation in load because of changes in dielectric properties of the material under treatment, are minimized. In use of this system characterized by avoidance of usual coupling loss and of necessity to retune the load circuit, it has been found possible substantially to increase the rate at which materials may be heated at frequencies up to about 100 megacycles. For higher frequencies and for reasons later discussed, other modifications hereinafter described are preferred.

It is to be understood that when it is desired to use frequencies lower than the natural period of the grid or anode line a suitable capacitymay be connected across either or both of these lines as required. When, for example, tubes 2| and 22 physical length of the vare of the 806 type, satisfactory operation at fre- I quencies as low as 5 megacycles may be attained by connection of capacities up to 500 micromicrofarads between the grid ends of conductors 2?, 28 and between the anode ends of conductors 23, 24. For the lower frequencies of this range, additional lumped inductance in series in the grid circuit is usually necessary to obtain ample excitation of the oscillator.

It is further to be understood that instead of the tuned-grid, tuned-plate long-lines oscillator above described in discussion of Fig. 1, there may be used any other type of oscillator having a tuned line in the anode circuit as a source of high frequency.

For frequencies above about 100 megacycles, the heating-electrode arrangement shown in Fig. 1 is responsible for an undesired shunt inductive effect on the anode line. For these higher frequencies, there should be used arrangements such as shown in Figs. 4 and 5, 6 and 7 providing for inductive coupling between the anode line and the line to which the heating-electrodes are attached: the same arrangement may be used of course at lower frequencies, particularly upwards of 50-megacycles.

'As shown in Figs. f and 5, the construction of the electrodes i0 and H themselves may be generally the same as above described in discussion of Figs. 2 and 3 but the method of mounting them is different: in this modification, they are supported upon a panel by suitable insulators 36, 31, 38 and 39: more specifically, the electrode ii! is at one end mechanically connected to insulator 38 by angle pieces 4!] and 42 and the corresponding end of electrode H is similarly mechanically connected to insulator 31 by angle pieces ii and 43. The other ends of the electrodes may, in generally like manner, be attached to insulators 38 and 33.

As shown, a test tube 46 may be disposed between electrodes [0 and H so that the concave electrode faces engage and partially encircle the test tube. When liquid is to be heated, the panel 35 is mounted in substantially vertical position with the out-turned lip of the tube 46 resting on the upper ends of the electrodes Ii! and ii.

The open ends of a hair-pin loop or trombone stub are slidably received by tubular members attached to or formed integrally with electrodes it and H to constitute therewith a line which by adjustment of slide 45 may be tuned to resonance at an odd number of quarterwavelengths at the operating frequency. In the arrangement shown in Figs. 4 and 5, the spaced, elongated electrodes by their disposition lengthwise of the line provide nearly the total inductance and capacity of the load circuit. The test tube being at the open end of the line is at a region of maximum voltage difference and since the electrodes closely conform to the test tube the electric field is to maximum extent utilized in dielectric heating of its contents.

In general this arrangement has the advantages of the system of Fig. 1 and in addition is better suited for operation at frequencies upwards of about 100 megacycles.

The electrode arrangement shown in Figs. 6 and 7 is suitable for operation at frequencies upwards from about 50 megacycles for the heating of materials in bottles, ampules and the like and, like the arrangement of Figs. 4 and 5, provides for inductive coupling to the anode line or circuit of a high-frequency oscillator.

The electrode and coupling assembly comprises the parallel, tubular conductors 33, 34, a hairpin loop 45 slidably received by one pair of adjacent ends of conductors 33, 34, and the heating electrodes 44, 41 attached respectively to the other ends of conductors 33, 34 substantially at right angles thereto. As shown, the electrodes 44, 4'! are shaped to fit or receive small bottles and the like for heating of their contents by action of the electric field between the electrodes.

As will be understood from prior explanation, the assembly constitutes a line which may be tuned by adjustment of loop 45 to resonance at an odd number of quarter-wavelengths of the oscillator frequency so that between electrodes 44 and 4'! there exists a maximum difference of radio-frequency potential.

For small containers 0r ampules, the electrodes 44, 41 are removed and replaced by the smaller electrodes 44a, 47a, Figs. 6A and 7A, so effectively to utilize the electric field at the electrodes. When such substitution is made the load circuit is retuned to compensate for change in electrical length of the line or less conveniently, the oscillator frequency is to suitable extent changed.

The length of each of the electrodes 44, 4 44a, lla should be much lessthan a quarter- Wavelength and the point of its connection to the line should be at or near the center of the electrode to ensure the voltage gradients from the connection to the edges of the electrode are as uniform as possible.

With the electrode arrangements previously described, the material is dielectrically heated in successive batches whereas the electrode arrangement shown in Figs. 8 to 10 provides for dielectric heating of material while in movement or flowing in a more or less continuous stream.

As in the arrangements of Figs. 4 and 5 and Figs. 6 and 7, the load circuit including the heating-electrode may be inductively coupled to a long lines oscillator. More specifically, the load circuit includes a loop 59 slidably received by couplings 66 integral with or extending from conductors 6'! individually connected to the electrodes B0 and iii shaped to lit and receive a funnel 62 of Pyrex glass, or other suitable insulation, and through which grain, cereal or the like may flow preferably at a rate controiled by valve 62a or equivalent. The funnel 52 serves as a container for the material being heated therein.

To minimize corona and permit operation at high voltages, the electrodes 39 and BI are pro vided with rings or guards 54 and 65 equivalent in function to the rounded elements 16 and I! of herein previously described modifications.

In practice, it has been found that dielectric heating of grains, cereals, various liquids and some gases may be effectively accomplished at rapid rate.

While several modifications of our invention have been specifically illustrated and described, it is to be understood that relatively wide variation may be made in the selection of circuit components and that the design of the electrodes may be altered to suit the requirements of the articles to be heated, all however within the spirit and scope of the invention as set forth in the appended claims: for example, in the modification shown in Figs. 5 to 10, it shall be understood the load line inductively coupled to the oscillator may be closed at both ends in which event its electrical length should be at even number of quarter-wavelengths for resonance with the heating electrodes at or adjacent a pair of voltage anti-nodes on the line conductors.

What we claims is:

1. A load circuit device to be inductively coupled to a high-frequency oscillator comprising two elongated conductors, heating electrodes respectively attached to the corresponding ends of said conductors substantially at right angles thereto and. shaped to support, to conform with and partially encompass articles to be heated, and means slidably engaging the other ends of said conductors to vary their length, said electrodes and conductors forming a line adjustable by said slidable means to electrical length corresponding with an odd number of quarter-Wavelengths and said electrodes being centrally connected to said conductors and much shorter electrically than a quarter-wavelength to provide for uniform voltage gradient along the electrodes.

2. A load circuit device to be inductively coupled to a high-frequency oscillator comprising two elongated spaced conductors, heating electrodes respectively electrically associated with the corresponding ends Of said conductors in spaced opposition and complementarily shaped to support, to conform with and partially to encompass a container for material to be dielectrically heated, and a conductive structure slidably engaging said pair of conductors to vary their length, said electrodes and conductors forming a line adjustable by said slidable conductive structure to electrical length corresponding with an odd number of quarter-wavelengths and said electrodes being centrally connected to said conductors and much shorter electrically than a quarter-wavelength to provide for uniform voltage gradient along the electrodes.

ROBERT B. PARMENTER. EDGAR L. MARVEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,503,224 Blaine July 29, 1924 1,555,258 Allcutt Sept. 29, 1925 1,972,050 Davis Aug. 28, 1934 2,048,316 Beatty July 21, 1936 2,124,012 Smith July 19, 1938 2,149,387 Brown Mar. 7, 1939 2,177,272 Zottu Oct. 24, 1939 2,303,341 Dufour et al. Dec. 1, 1942 2,336,555 Malling Dec. 14, 1943 2,370,423 Roberts Feb. 27, 1945 2,382,495 Mann et al Aug. 14, 1945 2,404,745 Roberts July 23, 1946 2,419,793 Rosencrans Apr. 29, 1947 2,422,525 Brown et al. June 17, 1947 2,434,330 Merz et al Jan. 13, 1948 2,442,114 Brown May 25, 1948 2,459,260 Brown Jan. 18, 1949 2,460,566 Brown et a1 Feb. 1, 1949 2,465,102 Joy Mar. 22, 1949 2,474,420 Himmel June 28, 1949 FOREIGN PATENTS Number Country Date 375,587 Great Britain June 30, 1932 417,564 Great Britain Apr. 18, 1934 557,731 Great Britain Feb. 28, 1945 OTHER REFERENCES Baker et al.: High-Frequency Heating of Conductors and Nonconductors, Electrical Engineering, February 1945, pages 54-56.

Madsen: Some Limitations of Dielectric Heating, Electrical World, Sept. 15, 1945, pages 94, 95. 

